1. Field of the Invention
The present invention relates in general to a master cylinder used as a hydraulic pressure source for a hydraulically operated braking device or other hydraulic actuators. More particularly, the invention is concerned with improvements in a combination of a tandem master cylinder wherein two presser pistons are disposed in tandem, and a hydraulically operated booster.
2. Description of the Prior Art
A tandem master cylinder equipped with a booster is widely used, for example, as a hydraulic pressure source for a hydraulically operated braking device for an automotive vehicle, wherein two mutually independent brake systems are provided. In this case, an operating force applied to a brake pedal or other operating member is boosted by the booster, and the thus boosted operating force acts on two presser pistons of the master cylinder, so that fluid pressures are produced in two mutually independent pressure chambers formed in front of the respective presser pistons. These fluid pressures are applied through mutually independent piping systems to brake cylinders provided for the wheels of a vehicle for restraining the rotation of the wheels.
In the master cylinder of the type indicated above, a large amount of brake fluid of a high pressure level may be supplied to the brake cylinders or other actuators, with a short operating stroke of the operating member and with a small operating force exerted to the operating member, if the presser pistons have a large pressure generating face, and the booster has a high boosting rate.
In the above case, however, there is an inconvenience when there arises a problem which causes a failure of supply of a pressurized fluid to the booster. Namely, if the booster fails to operate, the operating force applied to the operating member must be mechanically transmitted to the presser pistons of the master cylinder, so that the master cylinder may generate pressures in the pressure chambers. In the case where the presser pistons have a relatively large pressure generating face, an accordingly large force must be applied to the operating member, for the master cylinder to produce sufficiently high pressures. In this sense, there is an upper limit in the surface area of the pressure generating faces of the presser pistons of the master cylinder.
In view of the above inconvenience, there is proposed a brake system wherein a power chamber of a booster is connected, via changeover valves, to passages which connect the pressure chambers of a master cylinder to brake cylinders at the wheels of a vehicle, as disclosed in laid-open publication No. 57-939 of Japanese Patent Application (corresponding to German Patent Application No. P3016683.7). Normally, the changeover valves hold the brake cylinders in communication with the pressure chambers, but hold the brake cylinders disconnected to the power chamber of the booster. When a pressurized fluid is delivered from the power chamber while no pressure is generated in the pressure chambers of the master cylinder, the changeover valves are operated to allow the pressurized fluid from the power chamber to be supplied to the brake cylinders. This arrangement is based on a fact that until a brake clearance in each brake cylinder is eliminated, a relatively low pressure of the brake fluid is sufficient to operate each brake cylinder. If the load of return springs for the presser pistons of the master cylinder is set so that the presser pistons will not operate at such low pressure level, the brake cylinder may be operated with the brake fluid of a relatively low pressure which is produced in the power chamber of the booster as a result of an initial short operating stroke of a brake pedal, and which is delivered to the brake cylinders through the changeover valves. After the pressure in the power chamber exceeds an upper limit, the presser pistons of the master cylinder are activated to supply the brake cylinders with the brake fluid of a higher level which corresponds to the operating force applied to the brake pedal. In other words, a portion of a volume of the brake fluid necessary to activate the brake cylinders is delivered from the power chamber of the booster. In this arrangement, the required operating stroke of the brake pedal is reduced by an amount corresponding to the portion of the fluid received from the power chamber of the booster.
In the hydraulic brake device equipped with the changeover valves discussed above, the operating stroke of the brake pedal may be small only where the brake pedal is operated slowly enough to permit the presser pistons of the master cylinder to be activated after the brake clearance in each brake cylinder has been eliminated with the brake fluid delivered from the power chamber of the booster. However, if the brake pedal is operated abruptly and the presser pistons of the master cylinder are activated to deliver a pressurized fluid to the brake cylinders before the brake clearance in each brake cylinder has been completely eliminated, a portion of the fluid necessary for the elimination of the brake clearance is supplied from the master cylinder. This results in a corresponding increase in the required operating stroke of the brake pedal.